Welcome to BYC. It could be dehydration from hot weather or not drinking enough, or coccidiosis. Coccidiosis can be very common in pullets this age as they are developing resistance to cocci in the soil and droppings. Corid (amprollium) from feed stores cattle aisle, is a good treatment. Dosage is 1 1/2 tsp per gallon of water for 5 days. Symptoms are lethargy, hunched or puffed up posture, not eating, and runny droppings with mucus or blood. If she is dehydrated, give her water with electrolytes, such as SaveAChick.

I'm sorry for your loss. Please update your thread when you receive necropsy report. Hopefully, it wasn't anything contagious, such as Mareks or another virus. Necropsies are very helpful to others here to learn about what certain diseases look like. Was your pullet vaccinated for anything? Some viruses such as Mareks and lymphoid leukosis can cause poor immunity to common diseases, weight loss, and just a lack of vigor. Did she have any problems with lameness or wings drooping?